1. Field of the Invention
The present invention relates to a laser beam printer, facsimile apparatus, digital copier or similar image forming apparatus and more particularly to an optical scanning device advantageously applicable to a color image forming apparatus including a plurality of photoconductive elements.
2. Description of the Background Art
An optical scanning device of the type including a semiconductor laser or similar light source, a polygonal mirror or similar deflector and focusing optics is customary with an image forming apparatus. The deflector deflects a light beam issuing from the light source with its deflecting faces at a constant angular velocity. The focusing optics focuses the light beam deflected by the deflector on a desired surface in the form of a beam spot at a constant velocity. There is an increasing demand for higher image quality and lower cost in relation to this type of scanning device. One of the problems with this type of scanning device is irregular image density ascribable to shading.
Specifically, when the deflector deflects the incident light beam with its deflecting faces, the angle of the reflected light beam incident to the focusing optics continuously varies during each time of deflection. Reflection optics including mirrors for bending or returning an optical path are arranged between the deflecting face and the surface to be scanned in accordance with the layout of the optics. Further, a soundproof glass for preventing noise ascribable to the rotation of the deflector and a dustproof glass for protecting the optics from dust are often arranged between the deflecting face and the surface to be scanned.
The reflectance of light varies in accordance with the incidence angle, as well known in the art. It follows that the reflectance of the light beam deflected by the soundproof glass, dustproof glass and return mirrors continuously varies due to deflection. Consequently, the intensity of the deflected light beam to reach the desired surface continuously varies each time of deflection. Shading refers to the variation of the intensity of the beam spot occurring during each time of deflection, i.e., during one line of scanning. Shading renders the density of a recorded image irregular and thereby lowers image quality. This is particularly true with a halftone image.
It is an object of the present invention to provide an optical scanning device capable of effectively reducing the shading of optics included therein to thereby enhance image quality, and an image forming apparatus using the same.
An optical scanning device of the present invention includes a light source, a deflecting surface for deflecting a light beam output from the light source, focusing optics for focusing the light beam deflected by the deflecting surface on a subject surface to be scanned, and at least one reflecting member intervening between the deflecting surface and the subject surface. The light beam incident to the deflecting surface and the optical axis of the focusing optics are angularly spaced from each other in a plane of polarization. Assume that that when a section in the subscanning direction is seen from a side where the light beam is incident to the deflecting surface, the inclination of the reflecting member relative to a plane perpendicular to the light beam is positive when clockwise and is "THgr" (xc2x0), and that a function f(i) is f(i)=1 when i is an even number or f(i)=xe2x88x921 when i is an odd number. Also, assume that the direction of polarization of the light beam just after emission from the light source is inclined by an angle of xcex8 relative to the main scanning direction. Further, assume that when the clockwise direction as seen from the rear of the light source is positive, the angle xcex8 lies in a range of:
0xc2x0 less than xcex8 less than 45xc2x0 or xe2x88x9290xc2x0 less than xcex8 less than xe2x88x9245xc2x0xe2x80x83xe2x80x83(condition 1)
or
xe2x88x9245xc2x0 less than xcex8 less than 0xc2x0 or 45xc2x0 less than xcex8 less than 90xc2x0xe2x80x83xe2x80x83(condition 2)
Then, when the condition 1 holds, the reflecting member is the xe2x80x9cnxe2x80x9d reflecting member, as counted from the light source in the direction of propagation of the light beam, except for the deflecting surface and satisfies a relation of f(n)xc3x97"THgr" less than 0. When the condition 2 holds, the reflecting member is the xe2x80x9cnxe2x80x9d reflecting member, as counted from the light source in the direction of beam propagation, except for the deflecting surface and satisfies a relation of f(n)xc3x97"THgr" less than 0.